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||p.1 left column bottom paragraph:"For the purposes of this review, [investigators] define the ubiquitin system as the complement of proteins directly involved in ubiquitin ligation, deubiquitylation, and ubiquitin itself, leaving aside the specific substrates and those proteins that bind to them. One can think of it as a system that converts the input of free ubiquitin into a variable set of conjugated forms. In HeLa cells, the aggregate of ubiquitin system proteins is estimated to be ~1.3% of the total cellular protein (extracted from [primary source])."